Better set up of initial locale.

Rather than always installing en_GB.UTF-8 and setting it as the default,
we instead do the following:
* If the current LANG (perhaps passed in via SSH) is UTF-8, make sure it
  is generated, and remove any default locale setup if different;
* If it isn't, but the default locale is UTF-8, use that;
* If not, set up en_GB.UTF-8 as before.